It was an ergonomics issue. Despite (or maybe even because of) having big hands (~20cm), having index on R1, middle on R4 and ring on R5 caused too much strain because of the stretch I was having to do to between index and middle.
It's a huge shame, because I loved quite literally everything else about the controller. Very well made. Quiet shoulder buttons. I was looking forward to finding uses for the grip force. Touch pads were huge improvements over the original SC by using haptics to simulate clicks instead of the loud "clunk" of the original. Back buttons themselves felt great, too, with just the right amount of pressure needed to press to avoid accidental presses. It fixed everything I felt was wrong about the original SC.
If I thought I was going to use it enough in games that didn't have heavy R1 use but instead more touch pad for example, I'd have kept it. But I wanted it to be a primary controller for all situations, and for me it can't be that.
When I noticed the strain issue I went back to my 8bitdo Ultimate 2, which puts R4 next to R1/R2 instead. I rest my ring finger on the grip button differently (pressing more with the middle segment of the finger), and the strain issue went away immediately. Went back to SC again to see if it was just something to get over because it was new, but it happened again. Didn't want to waste £85 on something that caused me pain, so I accepted the disappointing truth.
For anyone that wants to visualise/feel what I mean, prime your hand to grip with the first 3 fingers holding a controller but placed on buttons. Notice the natural position each finger takes, with them not splaying much. Now spread the index apart from the others as much as you can and imagine repeated use of it in that position. It's not comfy. Ergonomically speaking, if you're going to encourage that I think the controller needs to be reshaped to encourage the natural position of the fingers.
This may be something that doesn't affect most people, and I don't want to discourage people based on my experience as I'm likely an outlier.
Personally, I'm now sticking to controllers with R4 up top. It's such a convenient placement (when done right) and allows for much more natural play control schemes. I consider that a stroke of genius, and a real "Why didn't anyone think of that before?" idea.